A flaw detector for detecting flaws present in a light-transmitting material is needed to detect flaws in a transparent glass plate manufactured on a transparent glass plate manufacturing line, for example, and feed back the detection results to the transparent glass plate manufacturing process to prevent the occurrence of flaws at the location of generation so as to improve product yield.
The flaw detector of the conventional type for transparent glass plates includes one type as disclosed in Japanese Patent Application Laid-open No. 29988 of 1976, in which the presence of a flaw in a glass plate is known by detecting only the reflected light of a projected light beam with a light receptor, and another type as disclosed in Japanese Patent Application Laid-open No. 1184 of 1976, in which the presence of a flaw in a glass plate is known by detecting only the transmitted light of a projected light beam with a light receptor.
The flaw detector disclosed in the aforementioned Japanese Patent Application Laid-open No. 29988 of 1976 can detect a flaw on the surface of a glass plate, but cannot detect a flaw inside the glass plate.
The flaw detector disclosed in Japanese Patent Application Laid-open No. 1184 of 1976, on the other hand, can detect a flaw inside a glass plate, but it is impossible or extremely difficult to detect a flaw on the surface of the glass plate.
The aforementioned flaw detectors cannot detect and identify the types of flaws, such as bubbles formed by air bubbles remaining inside the glass plate, foreign matter remaining inside the glass plate, knots formed by almost molten foreign matter remaining inside the glass plate in a shape having a streaming tail, or drips formed by the metallic tin content of tin bath deposited on the glass plate surface. Moreover, detection of an air bubble, foreign matter, etc. on the same detection level with a single light receptor could result in overly detection of the foreign matter and insufficient detection of the air bubble.
In addition, the use of the conventional flaw detector which cannot detect the size of a flaw accurately makes it impossible to adopt the so-called two-blade glass plate sorting system in which the quality of transparent glass plates cut in the manufacturing process is sorted into high and low grades.